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15.7.2 Executing One Instruction

Executing an instruction consists of calling the virtual method sp_instr::execute(), which is implemented for each instruction.

For instructions that can be executed directly, and don't depend on the evaluation of a general SQL statement or expression, the execution is very simple. See for example sp_instr_jump::execute(), sp_instr_hpush_jump::execute() or sp_instr_hpop::execute(). In all cases, the implementation of the execute() method is purely internal to the runtime interpreter.

For instructions that need to evaluate a general expression, like sp_instr_jump_if_not::execute(), or general instructions that need to execute an SQL statement, suchh as sp_instr_stmt::execute(), things are more complex. The implementation needs to leverage the existing code that is already capable of evaluating an expression or executing a query, and is implemented by the function mysql_execute_command().

The function mysql_execute_command(), for historical reasons (it was implemented before Stored Programs), is mostly designed to consume directly the result of the parser, which is passed as input in THD::lex.

To comply with this interface, the runtime for stored program has to provide a THD::lex structure before executing each instruction, to prepare an execution environment which looks as if the statement to execute was just parsed. Dealing with the existing interface for re-entrant execution of SQL statements is the role of the C++ class sp_lex_keeper. The wrapper method to used to execute instructions is sp_lex_keeper::reset_lex_and_exec_core(), which ultimately invokes the sp_instr::exec_core() instructions implementation.

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